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Does anyone have a good physical intuitive explanation for why laser and radar data bandwidth to space are such huge pipes? I have always been amazed at how sa

Does anyone have a good physical intuitive explanation for why laser and radar data bandwidth to space are such huge pipes? I have always been amazed at how satellites quote bandwidths on the order of Tb/s, when I don't fundamentally see how they're so different from gigabit fiber, copper, etc. You've still got some oscillation going on and a receiver that has to "decode" it just the same. Is there a physically intuitive way to understand why the bandwidth of these methods is so great?

adamsmith 6y ago
The higher the frequency the more data you can send. This is known as the Shannon Limit. https://en.m.wikipedia.org/wiki/Noisy-channel_coding_theorem https://en.m.wikipedia.org/wiki/Noisy-channel_coding_theorem
- shaklee3 6y ago
  Shannon limit says nothing about frequency. It's about bandwidth. You can have just as much sodium available at lower frequencies than higher, and in fact, it's usually preferable.
  - cozzyd 6y ago
    well kind of hard to find 1 GHz of bandwidth below 1 GHz in frequency .
    - shaklee3 6y ago
      The point being that people are discussing very high microwave frequencies Here. There is still a ton of spectrum available around 10-25GHz. You don't need 40+ to get that.
kbaker 6y ago
I don't think it is quite that high. Maybe on the order of 20 Gb/s. > Each satellite in the SpaceX System provides aggregate downlink capacity to users ranging from 17 to 23 Gbps, depending on the gain of the user terminal involved. Assuming an average of 20 Gbps, the 1600 satellites in the Initial Deployment would have a total aggregate capacity of 32 Tbps. https://fcc.report/IBFS/SAT-LOA-20161115-00118/1158349 https://fcc.report/IBFS/SAT-LOA-20161115-00118/1158349 They have a couple of GHz of bandwidth on the satellites for each of the user downlinks/ground station uplinks, and maybe a GHz for the user uplinks/ground station downlinks, operating in the Ku (12-18 GHz) and Ku (27-40 GHz) bands (requested frequencies on page 8 of the linked paper.)
kortex 6y ago
It's usually a straight dead shot. With optical fiber and any land based radio, there is always obstructions to bend around, physical loses, ground clutter. Basically signal to noise is way higher with line of sight. Higher SNR means lower bit error rate means deeper modulation (cutting edge is like 65536-QAM and maybe higher). Noisy channel coding theorem and all that.

adamsmith 6y ago

The higher the frequency the more data you can send. This is known as the Shannon Limit. https://en.m.wikipedia.org/wiki/Noisy-channel_coding_theorem https://en.m.wikipedia.org/wiki/Noisy-channel_coding_theorem

shaklee3 6y ago
Shannon limit says nothing about frequency. It's about bandwidth. You can have just as much sodium available at lower frequencies than higher, and in fact, it's usually preferable.
- cozzyd 6y ago
  well kind of hard to find 1 GHz of bandwidth below 1 GHz in frequency .
  - shaklee3 6y ago
    The point being that people are discussing very high microwave frequencies Here. There is still a ton of spectrum available around 10-25GHz. You don't need 40+ to get that.

kbaker 6y ago

I don't think it is quite that high. Maybe on the order of 20 Gb/s. > Each satellite in the SpaceX System provides aggregate downlink capacity to users ranging from 17 to 23 Gbps, depending on the gain of the user terminal involved. Assuming an average of 20 Gbps, the 1600 satellites in the Initial Deployment would have a total aggregate capacity of 32 Tbps. https://fcc.report/IBFS/SAT-LOA-20161115-00118/1158349 https://fcc.report/IBFS/SAT-LOA-20161115-00118/1158349 They have a couple of GHz of bandwidth on the satellites for each of the user downlinks/ground station uplinks, and maybe a GHz for the user uplinks/ground station downlinks, operating in the Ku (12-18 GHz) and Ku (27-40 GHz) bands (requested frequencies on page 8 of the linked paper.)

kortex 6y ago

It's usually a straight dead shot. With optical fiber and any land based radio, there is always obstructions to bend around, physical loses, ground clutter. Basically signal to noise is way higher with line of sight. Higher SNR means lower bit error rate means deeper modulation (cutting edge is like 65536-QAM and maybe higher). Noisy channel coding theorem and all that.